A bare ground evaporation revision in the ECMWF land-surface scheme: evaluation of its impact using ground soil moisture and satellite microwave data

摘要

In situ soil moisture data from 122 stationsacross the United States are used to evaluate the impact of a new bareground evaporation formulation at ECMWF. In November 2010, the bareground evaporation used in ECMWF's operational Integrated Forecasting System(IFS) was enhanced by adopting a lower stress threshold than for thevegetation, allowing a higher evaporation. It results in more realistic soilmoisture values when compared to in situ data, particularly over dry areas.Use was made of the operational IFS and offline experiments for theevaluation. The latter are based on a fixed version of the IFS and make itpossible to assess the impact of a single modification, while the operationalanalysis is based on a continuous effort to improve the analysis andmodelling systems, resulting in frequent updates (a few times a year).Considering the field sites with a fraction of bare ground greater than 0.2,the root mean square difference (RMSD) of soil moisture is shown to decreasefrom 0.118 m3 m?3 to 0.087 m3 m?3 when using the new formulation in offlineexperiments, and from 0.110 m3 m?3 to 0.088 m3 m?3 in operations. It also improvescorrelations. Additionally, the impact of the new formulation on theterrestrial microwave emission at a global scale is investigated. Realisticand dynamically consistent fields of brightness temperature as a function ofthe land surface conditions are required for the assimilation of the SMOSdata. Brightness temperature simulated from surface fields from two offlineexperiments with the Community Microwave Emission Modelling (CMEM) platformpresent monthly mean differences up to 7 K. Offline experiments with the newformulation present drier soil moisture, hence simulated brightnesstemperature with its surface fields are larger. They are also closer to SMOSremotely sensed brightness temperature.